Ch 24: Microbial Diseases of the Respiratory System

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Microbial disease of the upper respiratory system

  • Pharyngitis: Sore throat
  • Laryngitis
  • Tonsillitis
  • Sinusitis
  • Epiglottitis

Streptococcal pharyngitis (strep throat)

  • Caused by group A streptococci (GAS): Streptococcus pyogenes
  • Resistant to phagocytosis
  • Streptokinases lyse clots
  • Streptolysins are cytotoxic
  • Local inflammation, fever, tonsillitis, enlarged lymph nodes
  • Diagnosis by enzyme immunoassay (EIA) tests

Scarlet fever

  • Erythrogenic toxin produced by lysogenized
  • S. pyogenes


  • Caused by Corynebacterium diphtheriae: Gram-positive rod; pleomorphic
  • Prevented by DTaP vaccine
  • Forms a tough grayish membrane in the throat
  • Fibrin and dead tissue blocks passage of air to the lungs
  • Exotoxin produced by lysogenized bacteria
  • Circulates in the blood; damages the heart and kidneys

Otitis Media

  • Infection of the middle ear & formation of pus puts pressure on the eardrum
  • Causes: Streptococcus pneumoniae, Nonencapsulated Haemophilus influenzae, Moraxella catarrhalis, S. pyogenes, •Staphylococcus aureus
  • Common in childhood
  • Treated with broad-spectrum penicillins

The Common Cold

  • Over 200 different viruses
  • Rhinoviruses (30–50%)
  • Thrive in temperatures lower than body temperature
  • Coronaviruses (10–15%)
  • Sneezing, nasal secretion, congestion
  • Can lead to laryngitis and otitis media, not accompanied by fever
  • Antibiotics are of no use
  • Relief via cough suppressants and antihistamines

Microbial Diseases of the Lower Respiratory System

Caused by many of the same bacteria and viruses as the upper respiratory system

  • Bronchitis
  • Bronchiolitis
  • Pneumonia

Pertussis (Whooping Cough)

  • Caused by Bordetella pertussis: Gram-negative coccobacillus, capsule
  • Allows attachment to ciliated cells in the trachea & destroys ciliated cells and shuts down the ciliary escalator
  • Tracheal cytotoxin of cell wall damages ciliated cells
  • Pertussis toxin enters the bloodstream
  • Prevented by DTaP vaccine
  • Treated with erythromycin or other macrolides

Pertussis (Whooping Cough) stages

  • stage 1: catarrhal stage, like the common cold
  • Stage 2: paroxysmal stage, violent coughing, gasping for air
  • Stage 3: convalescence stage, may last for months


Caused by Mycobacterium tuberculosis: Acid-fast rod; obligate aerobe

  • 20-hour generation time
  • Lipids in the cell wall make it resistant to drying and antimicrobials

Pathogenesis of Tuberculosis

  • Inhaled organisms are phagocytized by alveolar macrophages
  • Mycolic acids in the cell wall stimulate an inflammatory response
  • Organisms are isolated in the walled-off tubercle•Tubercles heal and become calcified (Ghon's complexes)
  • Tubercle breaks down, releasing bacteria into the lungs and cardiovascular and lymphatic systems

Diagnosis of Tuberculosis

  • tuberculin skin test: Positive reaction means a current or previous infection
  • Followed by an X-ray or CT exam, acid-fast staining of sputum, and culturing of bacteria•New rapid blood test for IFN-γ and PCR test

Treatment of Tuberculosis

  • Minimum of 6 months of drug therapy due to slow growth and dormancy
  • First-line drugs: isoniazid, ethambutol, pyrazinamide, rifampin
  • Second-line drugs: aminoglycosides, fluoroquinolones, para-aminoslicyclic acid (PAS)
  • Multi-drug-resistant (MDR) strains: resistant to first-line drugs•Extensively drug-resistant (XDR) strains: resistant to second-line drugs

Bacterial Pneumonias

  • typical pneumonia: caused by S. pneumoniae
  • Atypical pneumonia
  • Lobar pneumonia: Infects the lobes of the lungs
  • Bronchopneumonia: Infects the alveoli adjacent to the lungs
  • Pleurisy: Pleural membranes inflamed

Pneumococcal Pneumonia

  • Caused by S. pneumoniae: Gram-positive; encapsulated diplococci
  • Infected alveoli of the lung fill with fluids and RBCs; interferes with oxygen uptake
  • Diagnosis: optochin-inhibition test, bile solubility test, or antigen in urine
  • Treated with macrolides and fluoroquinolones
  • Prevented with conjugated pneumococcal vaccine

Haemophilus influenzae Pneumonia

  • Gram-negative coccobacillus
  • Predisposing factors: alcoholism, poor nutrition, cancer, or diabetes
  • Symptoms resemble those of pneumococcal pneumonia
  • Diagnosis: isolation on special media for nutritional requirements (X and V factors)
  • Treated with cephalosporins

Mycoplasmal Pneumonia

  • Also called primary atypical pneumonia or walking pneumonia
  • Caused by Mycoplasma pneumoniae: No cell wall
  • Mild but persistent respiratory symptoms; low fever, cough, headache
  • Common in children and young adults
  • "Fried-egg" appearance on media
  • Treated with tetracyclines


  • Also called Legionnaires' disease
  • Caused by Legionella pneumophila: Aerobic, gram-negative rod
  • Grows in water and air conditioning, biofilms, and waterborne amebae
  • Transmitted by inhaling aerosols; not transmitted person to person
  • Symptoms: high fever and cough
  • Treated with erythromycin and macrolides

Chlamydial Pneumonia

  • Caused by Chlamydophila pneumoniae
  • Transmitted person to person
  • Mild respiratory illness common in young people; resembles mycoplasmal pneumonia
  • Possible association with artherosclerosis
  • Diagnosis: serological tests
  • Treated with tetracyclines

Psittacosis (Ornithosis)

  • Caused by Chlamydophila psittaci: Gram-negative intracellular bacterium
  • Transmitted to humans by elementary bodies from bird droppings transmitted through air
  • Fever, headache, chills, disorientation
  • Diagnosis: growth of bacteria in eggs, cell culture or PCR
  • Treated with tetracyclines

Viral Pneumonia

  • Viral pneumonia occurs as a complication of influenza, measles, or chickenpox
  • SARS-associated coronavirus (SARS
  • 2003•Middle East respiratory syndrome coronavirus (MERS-CoV)

Respiratory Syncytial Virus (RSV)

  • Most common viral respiratory disease in infants and life-threatening pneumonia in older adults
  • Almost all children are infected by age 2•14,000 deaths annually, mostly in older adults
  • Causes cell fusion (syncytium) in cell culture
  • Coughing and wheezing for more than a week
  • Diagnosis: serological test for viruses and antibodies
  • Treated with palivizumab

Influenza (Flu)

  • Chills, fever, headache, and muscle aches
  • No intestinal symptoms•
  • Avian, swine, and mammalian strains
  • Treated with zanamivir (Relenza) and oseltamivir (Tamiflu)


  • Minor antigenic changes in HA and NA
  • Allow the virus to elude some host immunity

Antigenic drift

  • Minor antigenic changes in HA and NA
  • Allow the virus to elude some host immunity

Antigenic shifts

  • Changes great enough to evade most immunity
  • Lead to pandemics
  • Involve the reassortment of the eight RNA segments


  • Also known as Valley fever or San Joaquin fever
  • Caused by Coccidioides immitis: Dimorphic fungus, Arthroconidia found in alkaline desert soils
  • Form a spherule filled with endospores in tissues
  • Most infections are not apparent; fever, coughing, weight loss•< 1% of cases resemble tuberculosis
  • Treated with fluconazole or itraconazole

Pneumocystis Pneumonia (PCP)

  • Caused by Pneumocystis jirovecii
  • No universal agreement if it is a protozoan or fungus
  • Asymptomatic in the immunocompetent; causes pneumonia in the immunocompromised
  • A primary indicator of AIDS
  • Found in the lining of the alveoli•Forms a cyst
  • Cysts rupture, releasing eight trophozoites
  • Treated with trimethoprim-sulfamethoxazole


  • Caused by Blastomyces dermatitidis
  • Dimorphic fungus: Grows in soil
  • Symptoms resemble bacterial pneumonia; cutaneous abscesses; extensive tissue
  • Treated with itraconazole and amphotericin B